Graphite intercalation and exfoliation process

ABSTRACT

The invention relates to expanded graphite and methods of making the graphite and products that can be made from the graphite made from the inventive process. The invention includes the step of introducing a fluid into at least one of a plurality of interstices of graphite flake, wherein the fluid comprises at least one of a sub-critical point fluid, a near critical point fluid, or a supercritical fluid. The graphite flake is also intercalated with an intercalant and optionally an oxidizing agent. The invention may further include novel techniques of exfoliating the graphite. The invention may be practiced to make nano-sized graphite particles and also graphite composites. Preferred composites which may be made in accordance with the invention include conductive polymeric composites (thermally or electrically), paint composites, battery composites, capacitor composites, and pollution abatement catalyst support composites.

FIELD OF THE INVENTION

[0001] This invention relates to graphite and more particularly to amethod of modifying graphite and creating novel graphite composites.

BACKGROUND OF THE INVENTION

[0002] Graphites are made up of layered planes of hexagonal arrays ornetworks of carbon atoms. These layered planes of hexagonally arrangedcarbon atoms are substantially flat, covalently bonded in the flatlayered planes, and are oriented or ordered so as to be substantiallyparallel and equidistant to one another. The substantially flat,parallel equidistant sheets or layers of carbon atoms, usually referredto as basal planes, are linked or bonded together and groups thereof arearranged in crystallites. Highly ordered graphites consist ofcrystallites of considerable size; the crystallites being highly alignedor oriented with respect to each other and having well ordered carbonlayers. In other words, highly ordered graphites have a high degree ofpreferred crystallite orientation. It should be noted that graphitespossess transversely isotropic structures and flexible graphite producedthus exhibit or possess many anisotropic properties that are highlydirectional, e.g., thermal and electrical conductivity and fluiddiffusion. Briefly, graphites may be characterized as laminatedstructures of carbon that are structures consisting of superposed layersor laminae of carbon atoms joined together by weak van der Waals forces.In considering the graphite structure, two axes or directions areusually noted, to wit, the “c” axis or direction and the “a” axes ordirection. For simplicity, the “c” axis or direction may be consideredas the direction perpendicular to the carbon layers. The “a” axes ordirection may be considered as the directions parallel to the carbonlayers or the direction perpendicular to the “c” direction. The naturalgraphites suitable for manufacturing flexible graphite possess a veryhigh degree of orientation.

[0003] As noted above, the bonding forces holding the parallel layers ofcarbon atoms together are weak van der Waals forces as compared to thecovalent bonds in the layered planes. Natural graphites can be treatedso that the spacing between the superposed carbon layers or laminae canbe appreciably opened up so as to provide a marked expansion in thedirection perpendicular to the layers, that is, in the “c” direction andthus form an expanded or intumesced graphite structure in which thelaminar character of the carbon layers is substantially retained.

[0004] With respect to the above treating of natural graphite, such asnatural graphite flake, with an intercalant, e.g., a solution ofsulfuric and nitric acid, the crystal structure of the graphite reactsto form a compound of graphite and the intercalant. The treatedparticles of graphite may be hereafter referred to as “particles ofintercalated graphite”. Upon exposure to high temperature, the particlesof intercalated graphite expand in dimension as much as 80 or more timesits original volume in an accordion-like fashion in the “c” direction,i.e., in the direction perpendicular to the crystalline planes of thegraphite. The exfoliated graphite particles are vermiform in appearance,and are therefore commonly referred to as worms. The worms may becompressed together into flexible sheets which, unlike the originalgraphite flakes, can be formed and cut into various shapes and providedwith small transverse openings by deforming mechanical impact.

[0005] A common method for manufacturing graphite sheet, e.g., foil fromflexible graphite, is described by Shane, et al. in U.S. Pat. No.3,404,061 the disclosure of which is incorporated herein by reference.As shown in FIG. 1, in the typical practice of the Shane, et al. method,natural graphite flakes 12 are intercalated 14 by dispersing the flakesin a solution containing a mixture of nitric and sulfuric acid. Theintercalation solution may contain acidic compounds and otherintercalating agents known in the art. Examples of acidic compoundsinclude solutions containing nitric acid, potassium chlorate, chromicacid, potassium permanganate, potassium chromate, potassium dichromate,perchloric acid, and the like, or mixtures, such as for example,concentrated nitric acid and chlorate, chromic acid and phosphoric acid,sulfuric acid and nitric acid, or mixtures of a strong organic acid,e.g., trifluoroacetic acid.

[0006] After the flakes are intercalated, any excess solution is drainedfrom the flakes and the flakes are water-washed. The quantity ofintercalation solution retained on the flakes after draining may rangefrom 20 to 150 parts of solution by weight per 100 parts by weight ofgraphite flakes (pph) and more typically about 50 to 120 pph.Alternatively, the quantity of the intercalation solution may be limitedto between 10 to 50 parts of solution per hundred parts of graphite byweight (pph) which permits the washing step to be eliminated as taughtand described in U.S. Pat. No. 4,895,713 the disclosure of which is alsoherein incorporated by reference. Upon exposure 16 to high temperature,e.g., 700° C. to 1000° C., the particles of intercalated graphite expandas much as 80 to 1000 times its original volume in an accordion-likefashion in the c-direction, i.e., in the direction perpendicular to thecrystalline planes of the constituent graphite particles. As previouslystated, the expanded graphite 18 is vermiform in appearance, and istherefore commonly referred to as worms. The worms may be compressedtogether into flexible sheets which, unlike the original graphiteflakes, can be formed and cut into various shapes and provided withsmall transverse openings by deforming mechanical impact as hereinafterdescribed.

[0007] A drawback of the above intercalation process is that it requiressignificant remediation of effluents generated during the process. Theprocess produces various species of sulfuric and nitrous compounds inboth liquid and gas phases that require remediation. There is a need todevelop an intercalation process that will reduce, preferably eliminate,the production of the environmentally unfriendly sulfuric and/or nitricspecies and likewise reduce, preferably eliminate, the use of chemicalcompounds to treat the environmentally unfriendly sulfuric and/or nitricspecies.

[0008] Another drawback of the intercalation and exfoliation process isthat the above process cannot be used to control the amount ofexfoliation or to produce nano-particle sized graphite flakes.Consequently, the expanded graphite flake produced from the process willhave a thickness of at least 10 microns or greater, typically at least50 microns or more. Therefore, a need also exists to be able to controlthe amount of exfoliation and to be able to produce nano-sized particlesof graphite.

SUMMARY OF THE INVENTION

[0009] A first aspect of the invention includes a method of modifyinggraphite. The method includes the step of introducing an intercalantinto at least one flake of natural graphite. The method further includesintroducing a fluid into at least one of a plurality of interstices ofthe flake. Preferably the fluid comprises at least one of a sub-criticalpoint fluid, a near critical point fluid, or a supercritical fluid.

[0010] Another aspect of the invention includes a method of making atleast one nano-sized flexible graphite particle. The steps of the methodinclude intercalating at least one flake of natural graphite with anintercalant and, optionally, an oxidizing agent, and introducing a fluidinto at least one of a plurality of interstices of the flake.Preferably, the fluid comprises at least one of a sub-critical pointfluid, a near critical point fluid, or a supercritical fluid.

[0011] A further aspect of the invention is a method of making agraphite composite. The method of making the graphite composite includesthe step of intercalating at least one flake of natural graphite with anintercalant and with or without an oxidizing agent. The method alsoincludes the step of introducing a fluid into at least one of aplurality of interstices of the flake. Preferably, the fluid comprisesat least one of a sub-critical point fluid, a near critical point fluid,or a supercritical fluid. The method further includes exfoliating thegraphite flake; thereby forming expanded graphite, and blending aneffective amount of the expanded graphite with a polymeric material,whereby forming the graphite-polymeric composite.

[0012] Furthermore, the aspects of the invention include agraphite-polymer composite. One composite includes a polymeric materialand an effective amount of a plurality of graphite particles such that aloading rate of the graphite particles in the composite comprises lessthan about 20%. Preferably at least one of the graphite particles has asurface area of at least about 100 m²/g or an aspect ratio of at leastabout 100,000:1.

[0013] One advantage of the invention is that the aforementioned methodsmay be used to manufacture graphite in a form that has a thickness ofless than about 10 microns, preferably less than about 1 micron, morepreferably less than about 100 nm, even more preferably less than about10 nm, and most preferably less than about 1 nm. Also, the inventivemethods may be used to increase the volume expansion of the graphiteparticle during processing as compared to the volume expansion resultingfrom the conventional intercalation-exfoliation process. The inventivemethods may result in a volume expansion of an individual graphiteparticle of more than about 1000 times. It is believed that the volumeexpansion of a particle processed by the inventive methods is at leastabout a ten (10) fold increase as compared to the volume expansion of aparticle treated with the conventional process.

[0014] Another advantage of the invention is that it includes a newmethod to process natural graphite into new forms of powdered graphitethat have applications in supercapacitors, batteries, catalyst supports,and polymeric/graphitic composites.

[0015] Practicing the invention will result in the advantage ofreducing, preferably eliminating, the production of the environmentallyunfriendly sulfuric and/or nitric species during the intercalationprocess. Likewise practicing the invention may result in the advantageof reducing, preferably eliminating, the use of chemical compounds totreat the environmentally unfriendly sulfuric and/or nitric speciesproduced during the intercalation process.

[0016] Another advantage of the invention, is that an aspect of theinvention may be practiced to produce an exfoliated graphite flake thatdoes not require the aforementioned heating of the intercalated graphiteto a temperature of 700° C. to 1200° C. A further advantage of theinvention is the invention may be practiced to produce a flexiblegraphite article that is substantially free of the aforementionedsulfuric species and/or nitric species. Furthermore, the advantages ofthe invention include that the invention enabled the use of an entirelynew class of chemical compounds as intercalates than what was previouslyunavailable as an intercalant.

[0017] Additionally, practicing the invention will result in theadvantage of being able to produce expanded graphite without having topractice traditional intercalation processes. The advantages of theinvention also include the ability to recycle the fluid, resulting inthe benefits of reducing the need for pollution abatement and reducingmaterial costs.

[0018] A further advantage of the invention is that the invention may bepracticed to separate adjacent graphene layers that do not have defectsbetween the adjacent layers, or defects within the crystalline layer.Defects is used herein to describe irregularities in the structure ofthe graphite flake such as crystalline dislocations, atomic vacancies,fracture planes, polycrystalline grain boundaries, or other crystallinestructure non-uniformities, as well as, impurities between the graphenelayers, such as the presence of silicon, magnesium, potassium, sodium,aluminum, iron, or phosphorus elements in between the graphene layers.

[0019] Furthermore, the advantages of the invention include the abilityto control the separation of the spacing between the respective graphenelayers of the graphite flake through the inventive intercalation andexfoliation processes. These processes may be used to produce anano-sized graphite particle. Nano-sized graphite particles haveadvantageous applications in the area of electrically conductive polymercomposites, thermally conductive polymer composites, supercapacitors,catalyst and/or metal supports, batteries, and microelectronics. Thenano-sized graphite particles can be added to polymeric materials toproduce composites with improved electrical properties, mechanicalstrength properties, coefficient of thermal expansion, and/or barrierperformance.

[0020] The advantages of the invention also include that the inventivemethods may be used to process graphite flake that has one dimension ofabout 50 microns or less, even graphite flake that has one dimension ofabout 10 microns or less.

[0021] Additional features and advantages of the invention will be setforth in the detailed description which follows, the claims, as well asthe appended drawings. It is to be understood that both the foregoinggeneral description and the following detailed description presentembodiments of the invention, and are intended to provide an overview orframework for understanding the nature and character of the invention asit is claimed. The accompanying drawings are included to provide afurther understanding of the invention, and are incorporated into andconstitute a part of this specification. The drawings illustrate variousembodiments of the invention, and together with the description serve toexplain the principles and operations of the invention

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is schematic diagram of an embodiment to make graphite inaccordance with the conventional intercalation and exfoliation process.

[0023]FIG. 2 is a schematic diagram of an embodiment to make graphite inaccordance with the invention.

[0024]FIG. 3 is an exploded schematic representation of the introductionof a supercritical fluid into the interstitial galleries of a piece ofnatural graphite flake in accordance with the invention.

[0025]FIG. 4 is a block diagram of two different stages ofintercalation.

[0026]FIG. 5 is a phase diagram for a fluid in arbitrary terms andshowing conditions for introducing the intercalant and the exfoliatingthe graphite in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The invention will be further described in regards to theaccompanying drawings. Whenever possible, like or the same referencenumerals may be used to describe like or the same elements. Illustratedin FIG. 2 is a schematic diagram of one embodiment of a method to modifygraphite in accordance with the invention, generally designated 20.

[0028] The method includes a step 24 of intercalating at least one flakeof natural graphite 22 with an intercalant and optionally an oxidizingagent. In one embodiment, the natural graphite flakes are intercalatedby dispersing the flakes in a solution containing a mixture of nitricacid (oxidizing agent) and sulfuric acid (intercalant). Theintercalation solution may contain other acidic compounds andintercalating agents instead of or in addition to sulfuric acid.Examples of other acidic compounds include solutions containing organicacids, e.g., acetic acid, nitric acid, potassium chlorate, chromic acid,potassium permanganate, potassium chromate, potassium dichromate,perchloric acid, and the like, or mixtures, such as for example,concentrated nitric acid and chlorate, chromic acid and phosphoric acid,sulfuric acid and nitric acid, or mixtures of a strong organic acid,e.g., trifluoroacetic acid. Optionally, the oxidizing agent may comprisenitric acid, ozone oxygen, hydrogen peroxide, bisulfate salts,carbonates, hexafluorophosphates, tetrafluoroborates, perchloarates, andcombinations thereof.

[0029] In an alternate embodiment, an electrolytic oxidation may be usedin place of, or along with, the oxidizing agent. For further descriptionregarding the use of an electrolytic oxidation the specification of U.S.Pat. No. 6,406,612 is incorporated herein by reference.

[0030] Preferred embodiments of the intercalating agent comprise asolution of a mixture of sulfuric acid, or sulfuric acid and phosphoricacid, and another component, i.e., nitric acid, perchloric acid, chromicacid, potassium permanganate, hydrogen peroxide, iodic or periodicacids, or the like. Although less preferred, the intercalation solutionmay contain metal halides such as ferric chloride, and ferric chloridemixed with sulfuric acid, or a halide, such as bromine in a solution ofbromine and sulfuric acid or bromine in an organic solvent.

[0031] Preferably, the intercalation comprises stage 5 or lower. Stage 5or lower is used herein to mean any one of stages 1-5 of intercalation.Therefore, the preferred stage of intercalation will bring about somedegree of separation between about every fifth layer of graphene toabout between each adjacent layer of graphene.

[0032] What is meant by a stage of intercalation is further illustratedin FIG. 4, generally designated 50. FIG. 4 illustrates the intercalationof a piece of natural graphite flake 52 to either stage 3 or stage 1intercalation. As shown in stage 3 intercalation 54, intercalant 60 isinserted between every third graphene layer 58. With respect to stage 1intercalation 56, intercalant 60 is inserted between each pair ofadjacent graphene layers 58.

[0033] Step 24, of FIG. 2, also includes introducing a fluid into atleast one of a plurality of interstices of the flake. A fluid is usedherein to define a compound that may be either a liquid, a gas, or asubstance near or above its critical point. Preferably the fluidcomprises at least one of a sub-critical point fluid, a near criticalpoint fluid or a supercritical fluid. A sub-critical point fluid has atleast one of a temperature or a pressure that is less than a criticalpoint for the fluid, preferably both. With respect to FIG. 5, at leastone of a temperature or a pressure comprises less than the respectivevalues associated with critical point 74, preferably both thetemperature and the pressure.

[0034] A near critical point fluid is a fluid that has temperature thatcomprises at least the temperature of the triple point for theparticular fluid and a pressure that comprises at least the pressureassociated with the triple point for the fluid. However, in thecombination of the pressure and temperature, at least one of thepressure or the temperature is no more than about the critical point ofthe fluid. With respect to FIG. 5, at least one of the pressure ortemperature is up to at least point 72 for the triple point and no morethan about point 74 for the critical point.

[0035] A supercritical fluid may be commonly known as a liquid with theproperties of a gas or vice versa. The properties of a supercriticalfluid are a combination of the properties while the fluid is in the gasstate and the liquid state. In other words, some of the properties ofthe fluid will correspond to the fluid in the liquid state, while otherswill correspond to the fluid in the gas state. With respect to FIG. 5,the temperature and pressure of the fluid comprise at least atemperature or pressure associated with the critical point 74 orgreater.

[0036] Preferred types of fluids are water, helium, and carbon dioxide.The critical point (also referred to as the supercritical point) ofwater comprises a temperature of at least about 374° C. and a pressureof at least about 22.1 MegaPascals. At or about the critical point, thedensity and viscosity of water decreases, increasing the diffusivity ofthe water molecules and the mobility of other chemical species dissolvedin the water. Consequently, an improvement in mass transfer is achieved.

[0037] Also, the acid product changes and the dielectric environment andthe oxidizing power of the medium are intensified. All of theseproperties contribute to making supercritical water an excellentreaction medium. The dielectric constant of supercritical waterdecreases from 78 to 5. The aforementioned dielectric constant is atypical value for polar constants, thus making the water a good solventfor gases and organic compounds. In light of the above changes inproperty, supercritical water has demonstrated great ease in penetratinga porous structure. The critical point of carbon dioxide comprises atemperature of at least about 31° C. and a pressure of at least about7.4 MegaPascals. Supercritical carbon dioxide has exhibited a similarbehavior as super critical water and has also demonstrated great ease inpenetrating a porous structure. In addition to supercritical water andsupercritical carbon dioxide, other suitable supercritical fluidsinclude supercritical helium and supercritical water oxidation.Supercritical water oxidation is commonly referred as water with a highoxygen content. The critical point of supercritical water oxidation isabout 500° C. and about 700 bar pressure.

[0038] It is preferred that the intercalant and oxidizing agent issoluble in the fluid, but, the properties of the intercalant andoxidizing agent may be tailored to a multistep processes. Multistepprocess is used herein to describe at least when intercalation andexfoliation do not have the same temperature and pressure conditions,such as shown in FIG. 5. Any combination of intercalation-exfoliationtemperatures and pressures may be used in practicing the invention. Forexample, the introducing of the intercalant may occur at a temperatureand pressure in the sub-critical point range. The temperature andpressure may be subsequently changed to above the critical point and theflake exfoliated.

[0039] Two examples of how the fluid may be introduced into the flakeare cited below. The examples are meant only to be demonstrative and notmeant to limit the invention. In one example, the fluid is introducedinto a vessel containing the flake. The vessel is then heated andpressurized to about near critical conditions of supercriticalconditions. Then the intercalant and oxidizing agent are added to thevessel. In a second example, the flake is introduced into a vessel,already substantially at or near the supercritical conditions,containing the fluid. After the flake is introduced into the vessel, theintercalant and oxidizing agent may be charged into the vessel.

[0040] An advantage of intercalating the natural graphite andintroducing the fluid into the graphite is that the fluid is introducedinto non-defect areas of the interstices of the graphite. This meansthat the fluid can be located between graphite layers that do notinclude the aforementioned compositional or structural defects betweenthe graphene layers.

[0041] With respect to the timing of the introducing steps of theintercalant and the fluid, optionally, the two steps may be started atsubstantially the same time. Alternatively, the introducing of the fluidstep may be started after introducing the intercalant step has beenstarted. In a third embodiment, the introducing of the fluid step may bestarted after the introducing of the intercalant has been substantiallycompleted. In a fourth embodiment, introducing the fluid step may beinitiated prior to introducing the intercalant step.

[0042] An example of the introduction of the fluid into the graphiteflake is depicted in FIG. 3. FIG. 3 is an exploded schematicrepresentation of the introduction of a supercritical fluid into theinterstitial galleries of a piece of natural graphite flake inaccordance with the invention, generally designated 40. Depicted in FIG.3 are a plurality of layered planes of carbon atoms 42 which make up agraphite flake 41. Flake 41 includes a carbon atom at each junction 43where two lines meet. The interstitial galleries of flake 41 arerepresented by the area between layered planes 42 in the c direction offlake 41. Further illustrated in FIG. 3, is the introduction of a fluid46 into the interstitial galleries (interstices) 48 of graphite flake 41through the use of an introduction element 44.

[0043] With respect to function, the use of the intercalant and theoxidizing agent does not achieve the same end result as the fluid. Theoxidizing agent, if used, and the intercalant substantially react inconcert to open up the graphene layers of the graphite flake. The fluidpenetrates the interstitial galleries of the flake. A main function ofthe fluid is not to substantially react with the flake, but tosubstantially expand the distance between the graphene layers duringsubsequent exfoliation.

[0044] One difference between the invention and the conventional processcan be seen in comparing FIGS. 1 and 2. In FIG. 1, graphite 12 isintercalated 14 and exfoliated 16 to arrive at expanded graphite 18. Asshown in FIG. 2, graphite 22, which is intercalated 24 with the fluidand exfoliated 26, has a greater capacity to expand. As depicted in FIG.2, graphite 22 may be expanded into individual graphene layers 28 withthe inventive process.

[0045] Back to introducing the intercalant step, the temperature of theintercalation step may comprise a temperature associated with asub-critical point and up to or above a temperature associated with thesupercritical point (also referred to as the critical point) of thefluid and anywhere in between. The pressure of the intercalation stepmay comprise at least a pressure associated with a sub-critical pointfluid and up to or above a pressure associated with a supercriticalpoint of the fluid and anywhere in between. Any combination oftemperature and pressure conditions can be used for the step ofintroducing the intercalant. As shown in FIG. 5, preferred intercalationtemperature and pressure conditions can include intercalating the flakeunder near critical point conditions, e.g., about triple point to aboutthe supercritical point, between points 72 and 74.

[0046] Optionally, the inventive method may further comprise step 26 ofexfoliating the graphite. Preferably, exfoliation will result in furtherseparation of the graphene layers 28. In one embodiment, the exfoliationstep comprises heating the flake to at least about 700° C. andpreferably, no more than about 1200° C. An additional embodiment ofexfoliation step 26 comprises the introducing step occurring in apressurized vessel having an internal pressure of greater thanatmospheric pressure and the exfoliating step comprises reducing theinternal pressure of the vessel. Preferably, the pressure in the vesselis reduced to atmospheric pressure. Furthermore, exfoliating step 26 maycomprise reducing the pressure in the vessel by discharging the flakewith the fluid from the vessel. Spraying the flake from the vessel isone technique that may be used to discharge the flake from the vessel.Optionally, the flake may be sprayed onto a substrate, into a secondvessel, or recycled into the same vessel.

[0047] In another embodiment of exfoliating step 26, it is preferred,that step 26 comprises exfoliating the flake at a maximum temperature ofthe flake of less than about 700° C., more preferably the flake does notachieve a temperature associated with a supercritical point of thefluid. Preferably, this embodiment of exfoliating step 26 does notinclude heating the flake to a temperature above the present operatingconditions of the aforementioned method. In a further embodiment,exfoliating step 26 comprises increasing the volume of the fluid in theinterstices of the flake. Preferably, the volume of the fluid isincreased rapidly, such that the expansion of the fluid in theinterstices functions similarly to a fluid boiling inside theinterstices and results in further separation of the layers of grapheneadjacent the fluid.

[0048] As shown in FIG. 5, one preferred embodiment of exfoliation step26 is to exfoliate the flake at temperature and pressure conditions thatare at or above the supercritical point of the fluid. This is atemperature and pressure at point 74 of FIG. 5 or higher. Preferably,the steps of introducing the intercalant and the fluid 24, andexfoliating 26 all occur in the same reactor. It is also preferred thatexfoliation is accomplished in a single pass. Meaning that theexfoliated particles do not have to be collected and be processedthrough more than one exfoliation step. In one embodiment the steps ofintroducing the intercalant and the fluid 24 and exfoliation 26 occursubstantially simultaneously. One technique to accomplish this is thatall three steps occur at the end of a spraying apparatus. This techniquemay further include recirculating the flake or the fluid into anothervessel or the same vessel.

[0049] The inventive method may also include a recycling step of thefluid for use in subsequent introducing steps. Preferably, the recyclingstep comprises collecting the fluid after exfoliation and using thecollected fluid with non-collected fluid for a subsequent introducingstep. Optionally, recycling may include storing the collected fluid. Themethod may also include washing the flake with water, preferably beforethe exfoliation step.

[0050] The invention further includes another method of forming theinventive graphite. The method includes the step of increasing thelength of at least one interstice between at least two adjacent layersof at least one flake of natural graphite through an oxidation-reductionprocess. The method further includes introducing the fluid into the atleast one interstice of the flake and expanding a volume of the fluid inthe interstice.

[0051] The aforementioned inventive methods may be used to formnano-sized expanded graphite particles. A nano-sized particle has atleast one dimension that is less than about 1 micron. Preferably, thenano-sized particle has at least one dimension of less than about 100nm, more preferably less than about 50 nm. In preferred embodiments ofthe nano-sized particle graphite, preferably the thickness of theparticle is less than about 10 nm, more preferably less than about 5 nm,even more preferably less than about 1 nm, and most preferably less thanabout 0.5 nm. For example, the particle may comprise a single graphenelayer. It is also preferred that the graphite particle has a surfacearea of at least about 100 m²/g, more preferably at least about 500m²/g, even more preferably at least about 1000 m²/g, most preferably atleast about 2000 m²/g. With respect to an aspect ratio, preferred aspectratios comprise at least about 20,000:1, preferably at least about100,000:1, and most preferably at least about 200,000:1.

[0052] High Resolution Transmission Electron Microscopy (HRTEM) andScanning Electron Microscopy (SEM) can be utilized to verify the aspectratio of graphite particles generated. Density measurements can give anindication of exfoliation characteristics, X-Ray Diffraction (XRD) canalso be utilized to indicate the extent of graphene layer separation.

[0053] The exfoliated graphite may be used to manufacture various typesof composites. Composites manufactured with the inventive graphite haveimproved mechanical strength, modulus, barrier performance, thermalexpansion, electrostatic discharge (ESD) (surface/volume resistivity ofabout 10⁵ to about 10¹² ohms/sq.), electromagnetic interference (EMI)shielding (surface to volume resistivity about 1 to about 10⁵ ohms/sq.),and electronic thermal management (ETM) properties, high electron orhole mobility flexible substrates, and a reduction in weight. In thebelow composites, the inventive graphite particles may be in the form ofa powder. A powder is used herein in terms of ASTM B 243(95) meaningthat the particle has at least one dimension of about 1000 microns orless.

[0054] One such composite is a graphite-polymer composite. The compositeis manufactured by blending an effective amount of the graphite with apolymeric material, thereby forming the graphite-polymeric composite.Preferably the loading rate of the composite comprises less than about20%. Preferably, the loading rate comprises about 15% or less, morepreferably about 10% or less, even more preferably less than about 5%,and most preferably, less than about 3%. The loading rate is describedherein to mean the percent by weight of the amount of graphite in thecomposite. Examples of suitable polymers for the composite includenylons, polyvinyl chloride, poly(methyl)methacrylates, polystyrene,polyethylenes, polypropylenes, polystyrenes, polycarbonates, epoxies,polyfluorinated hydrocarbons, e.g., perfluorinated hydrocarbons,polyimides, polyamides, fluorinated polymers, acryloides, polyacrylics,polyesters, cyanate esters, bismal imides, hydrophobic polymers, andcombinations thereof. Preferably at least one of the graphite particlesin the composite has an aspect ratio of at least about 100,000:1, morepreferably at least about 200,000:1. It is more preferred that amajority of the particles which make-up the composite have theaforementioned aspect ratios.

[0055] Graphite particles made in accordance with the invention may alsobe used in a composite to form a capacitor, preferably a double layercapacitor. Preferably, the capacitor composite comprises a plurality ofexpanded graphite flakes and a polymeric material. A preferred loadingrate of the graphite is at least about 70%, more preferably at leastabout 80%, even more preferably at least about 90%, and most preferablyat least about 95%. Preferably, the composite comprises at least onegraphite particle having a surface area of at least about 1000 m²/g.More preferably, the surface area comprises at least about 2000 m²/g. Itis further preferred that at least about one-fourth of the particles inthe composite has the aforementioned surface area, more preferably, atleast about one-half. A preferred polymeric material for the compositioncomprises poly(vinylidene fluoride) and related polymers, such as blockcopolymers which poly (vinylidene fluoride) comprises one or more of theblocks. Preferred solutions for the double layer capacitor compriseorganic solutions, which comprise carbonates, e.g., acetonitrile orwater based solutions with sulfuric acid or potassium hydroxide. Onepreferred solution is a mixture of tetraethylamonium andtetrafluoroborate. Typical voltage requirements for a double layercapacitor comprise at least about 1 volt, more preferably at least about2 volts. It is also preferred if the double layer capacitor has auseable energy density of about 1 Wh/kg and a power pulse density of atleast about 1 kW/kg. For additional description regarding capacitors thespecification of U.S. patent application Ser. No. 10/022,596 filed on orabout Dec. 31, 2001 is incorporated herein by reference in its entirety.

[0056] Graphite particles made in accordance with the invention may alsobe used in a paint composite. Preferably, an effective amount of theexfoliated graphite particles are blended with a paint to form acomposite, wherein the paint composite has sufficient electricalconductivity and viscosity for use in a spray paint booth.

[0057] The inventive graphite may also be used to form a pollutionabatement catalyst support composite. The catalyst support composite maybe formed by blending an effective amount of the exfoliated graphitewith a hydrophobic polymer to form the composite. In one embodiment ofthe composite, carbon black is blended into the composite. Optionally, ametal may be deposited onto a surface of the composite. A preferredtechnique for depositing the metal is electroless plating or vapordeposition. Preferred metals include iron, nickel, platinum, stainlesssteel and titanium. The catalyst support maybe attached to a catalyst.Preferred types of catalyst include transition metals which includesbinary and tertiary alloys of such transition metals. Preferred metalsinclude platinum, osmium, ruthenium, and combinations thereof. Thecatalyst support has applications in the areas of hydrogenationreactions and petroleum refining.

[0058] Graphite made in accordance with the invention may also be usedin a battery. The inventive graphite may be used to improve conductivityand connectivity of the internal components of the battery, which willimprove the useful life of the battery. In the case of an alkalinebattery, the inventive graphite particles may be attached to thecathode. In the case of a lithium ion battery, the inventive graphiteparticles may be attached to the anode.

[0059] The inventive graphite also has applications in the field ofmicroelectronics in an area such as thin films. An example of a thinfilm is a polyimide film of about 200 microns thick. Preferably, thepolyimide film further comprises an effective amount of a polymer sothat the film has excellent flexibility properties for the applicationof the film. Preferred polymers include polyimides, polyhydrocarbons,e.g., polyethylene and polyfluorinated hydrocarbons, perfluorinatedhydrocarbons. It is also preferred that the film comprises an effectiveamount of the inventive graphite. A factor in determining the effectiveamount of graphite is the desired electron or hole mobility of the film.Electron or hole mobility is herein defined as the ability to moveelectrons or holes (the absence of electrons) in the film. The inventivefilm will have applications in products such as liquid crystal displaydevices, transistor, memory elements, and logic elements for signalprocessing.

[0060] With respect to the aforementioned graphite-composites, thegraphite may be blended into the polymeric material at any time duringthe manufacturing process. For example, the graphite may be blended intothe polymeric material after exfoliation. Alternatively, the graphitemay be blended into the polymeric material during prior to exfoliation.This flexibility in processing may lead to an in-situ polymerizationprocess.

[0061] In the aforementioned composites, it is understood that thecomposites comprise preferably at least about one-quarter graphiteparticles made in accordance with the invention, more preferably atleast about one-third, even more preferably at least about one-half, andmost preferably substantially all.

[0062] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of the invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of modifying graphite comprising:introducing an intercalant into at least one interstice of at least oneflake of natural graphite; and introducing a fluid into said at leastone interstices of said flake, wherein said fluid comprises at least oneof a sub-critical fluid, near critical point fluid, or a supercriticalfluid.
 2. The method according to claim 1 wherein said fluid comprisesat least one of carbon dioxide, water, and helium.
 3. The methodaccording to claim 1 wherein said introducing said intercalant furthercomprises introducing an oxidizing agent.
 4. The method according toclaim 1 further comprising heating said flake to a temperature of about700° C. to about 1200° C.
 5. The method according to claim 1 whereinsaid introducing of said fluid occurred in a pressurized vessel havingan internal pressure greater than atmospheric pressure and furthercomprising reducing said internal pressure of said vessel.
 6. The methodaccording to claim 1 further comprising exfoliating said flake at amaximum temperature of less than about 700° C.
 7. The method accordingto claim 1 wherein said fluid in said interstices of said flake occupiesa predetermined volume within said interstices, further comprisingincreasing said volume occupied by said fluid.
 8. The method accordingto claim 1 wherein a temperature of said introducing of said intercalantcomprises at least a temperature associated with a triple point of saidfluid and no more than a temperature associated with a supercriticalpoint of said fluid.
 9. The method according to claim 1 wherein apressure of said introducing of said intercalant comprises at least apressure associated with a triple point of said fluid and no more than apressure associated with a supercritical point of said fluid.
 10. Themethod according to claim 1 wherein a temperature of said introducing ofsaid intercalant comprises at least a temperature associated with asupercritical point of said fluid and a pressure associated with saidsupercritical point of said fluid.
 11. The method according to claim 5wherein said discharging comprises spraying said flake and said fluidfrom said vessel.
 12. The method according to claim 1 further comprisingexfoliating said flake and wherein said steps of introducing saidintercalant, introducing said fluid, and said exfoliating occursubstantially at about the same time.
 13. The method according to claim3 wherein said oxidizing agent comprises ozone.
 14. The method accordingto claim 1 wherein said fluid comprises a supercritical fluid.
 15. Themethod according to claim 5 wherein said reducing said pressurecomprises recirculating said flake and said fluid into said vessel. 16.The method according to claim 1 further comprising blending an effectiveamount of said flake with a polymeric material, thereby forming agraphite-polymeric composite having a loading rate of less than about20%.
 17. The method according to claim 1 wherein said intercalating stepcomprises no more than about stage 5 or lower.
 18. The method accordingto claim 1 further comprising washing said graphite.
 19. A method ofmaking at least one nano-sized flexible graphite particle comprising:introducing into at least one flake of natural graphite an intercalant;and introducing a fluid into at least one of a plurality of intersticesof said flake, wherein said fluid comprises at least one of asub-critical point fluid, a near critical point fluid, or asupercritical fluid.
 20. The method according to claim 19 wherein saidfluid comprises at least one of supercritical water oxidation,supercritical carbon dioxide, supercritical helium, and supercriticalwater.
 21. The method according to claim 19 further comprising heatingsaid flake to a temperature of about 700° C. to about 1200° C.
 22. Themethod according to claim 19 wherein said introducing of said fluidoccurred in a pressurized vessel having an internal pressure of greaterthan atmospheric pressure and farther comprising reducing said internalpressure of said vessel.
 23. The method according to claim 19 furthercomprising exfoliating said flake at a maximum temperature of less thanabout 700° C.
 24. The method according to claim 19 wherein said fluid insaid interstices of said flake occupies a predetermined volume withinsaid interstices, further comprising increasing said volume occupied bysaid fluid.
 25. The method according to claim 19 wherein a temperatureof said introducing of said intercalant comprises at least a temperatureassociated with a triple point of said fluid.
 26. The method accordingclaim 25 wherein a pressure of said introducing of said intercalantcomprises more than atmospheric pressure and up to about a pressureassociated with a triple point of said fluid.
 27. The method accordingto claim 22 wherein said reducing said pressure comprises dischargingsaid flake and said fluid from said vessel.
 28. The method according toclaim 19 further comprising blending an effective amount of said flakewith a polymeric material, thereby forming a graphite-polymericcomposite having a loading rate of less than about 20%.
 29. The methodaccording to claim 1 further comprising blending an effective amount ofsaid flake with a polymeric material, thereby forming agraphite-polymeric composite having a loading rate of at least about70%.
 30. The method according to claim 19 further comprising blending aneffective amount of said flake with a polymeric material, therebyforming a graphite-polymeric composite having a loading rate of at leastabout 70%.